This invention relates to the mixing and melting of plastic material in an extruder and, more particularly, to the screw which performs these operations in the extruder.
In thermoplastic extruders, screws are used to mix, plasticize and convey the platic material axially along the cylinder bore from the point where the plastic enters in the form of pellets or other solid form to the other end where the plastic exits in the form of a liquid melt. Conventionally, these functions are performed by varying the pitch, thickness and lead of the screw threads at spaced intervals along the length of the screw. The root diameter of the shaft portion of the screw is sometimes also varied uniformly to increase or decrease the cross sectional area and, therefore, the pressure within an axial section along the screw length. Thus, the plastic is mixed, melted and conveyed along successive sections of the screw length. Examples of some typical extruder screw configurations are illustrated in U.S. Pat. Nos. 3,197,814; 3,486,192 and 3,023,456.
However, it has always been difficult to mix the plastic thoroughly to continuously melt new solid material as the pellets tend to form a solid plug adjacent to the screw shaft as they enter the extruder and where they are inclined to remain as they come under the pressure of the operating screw. The rotating screw thread wipes against the extruder bore wall. The shearing action of the plug compressed against the bore wall creates heat which melts off the solid plug surface along the threads ' outer peripheral surface. The melted plastic becomes susceptible to being damaged by overheating unless it can be conveyed away and be replaced by an unmelted portion of plastic material. Some prior screw designs attempt to expedite the melting process by inserting a fluted section in the screw to force the plastic over a plurality of axially extending dam-like radial edges to increase the shearing action on the plastic to promote raising its temperature and shorten the melting time. Other prior art screws incorporate reverse threads along a section to reverse the flow and increase the mixing action. But such devices only operate on a relatively small portion of the material at a time and still represent a separation of the mixing, plasticizing and conveying, or pumping, steps. Further, the fluted, or dam-type, screws tend to work satisfactorily only after the plastic has already been partially melted.